1. Field of the Invention
The present invention relates to a method of and an apparatus for recording halftone images to be employed in reproducing a color image, and more particularly, it relates to a method and an apparatus which can prevent a rosette moire from appearing in the color image. The present invention also relates to halftone images produced thereby.
2. Description of the Prior Art
In a printing process for reproducing a color image having continuous changes of color tone, a plurality of color separation images are generated from an original color image. The color separation images are recorded on a photosensitive film or the like as halftone images which represent density distribution of the respective color separation images by means of the size of halftone dots.
The halftone images are formed while employing respective screen angles different from each other to prevent a moire from appearing in the reproduced color image. For example, screen angles of 0.degree., 75.degree., 15.degree. and 45.degree. are employed in the halftone images of yellow magenta, cyan and black printers, respectively.
There are some techniques well known in the art about setting respective screen angles for the halftone images. First, Japanese Patent Publication Gazette 52-49361 discloses a so-called rational-tangent method. The method is named so because a tangent of a screen angle is a rational number. Second, Japanese Patent Publication Gazette 55-6393 discloses a so-called irrational-tangent method in which a tangent of a screen angle is an irrational number.
According to the prior art, a rosette moire (or a rosette pattern) appears in a reproduced color image which is printed on the basis of three halftone images having respective screen angles 15.degree., 45.degree., and 75.degree., for example. The rosette moire is a pattern of circles located around a particular position where respective halftone dots of the three halftone images are overprinted. The rosette moire can be seen with the naked eye and is accepted to be deterioration of image quality because the rosette moire usually includes a great many circles. Therefore, a method of recording halftone images which can prevent the rosette moire has been an issue in this field.